This article from NewScientist discusses how phytoplankton are disappearing — and they provide half of the food animals both in the ocean and on land depend on, plus produce a great deal of the oxygen we breathe.

Phytoplankton levels have dropped by almost 40 per cent since the 1940s (Worm, 2010. (Nature, vol 466, p 591).

Their numbers have declined in 8 out of 10 ocean regions at about 1 per cent per year over the last 40 years.

Why?

What we do know is that in many parts of the oceans, phytoplankton growth is limited by a lack of “fertilizer” – vital nutrients such as nitrate, phosphate and iron.

Upwelling of deep water and big storms that stir the oceans are the main source of these nutrients.

What makes Worm’s decline so scary is that it seems to be happening worldwide at the same time.

The obvious suspect is global warming. More than 90% of the heat retained by Earth as a result of rising greenhouse gases ends up in the sea. Plankton do grow faster in warmer conditions, but warming has a far less desirable effect, too. As surface waters warm, they become less dense and this makes it harder for cold, nutrient-rich water to rise to the surface. Less mixing means less fertilizer, and if phytoplankton run out of nutrients they cannot grow however warm the water is. So on balance, warmer waters are expected to reduce phytoplankton growth, and this is just what Worm’s team found. Apart from in the Arctic and Southern oceans, there was a strong link between higher sea surface temperatures and lower phytoplankton levels.

Oceanographers almost all agree that warming will lead to a decline in phytoplankton, but most expected only a small fall over the coming decades. And while there have already been dramatic falls in fish catches in many parts of the world, these have been attributed to overfishing rather than falling phytoplankton.

Other scientists disagreed with these numbers, so Worm and his team went back to their original data, and pulled in other data from scientific teams studying this issue. Their initial results still point to a worldwide decline of somewhere between 20 and 70 per cent.

“From everything we have done so far, we’re seeing a decline,” says Worm. “No matter what we include or exclude, we are always seeing a decline. The magnitude of the decline, and the regional detail, is still in question – but that there is a decline, I have very little doubt.”

A similar study in 2006 came to much the same conclusions. “What we see in the satellite record, very clearly, is there is a very strong relation between climate-driven changes in the surface temperature and the plankton,” says team member Michael Behrenfeld of Oregon State University in Corvallis.

The Future

There’ll be winners and losers because of big regional differences — phytoplankton levels rose in two-fifths of the ocean.

The bad news is that even in areas where productivity rises, there will not necessarily be more fish in the sea. In temperate regions, the phytoplankton tends to consist of large cells that are eaten by large zooplankton, such as copepods, and then by fish. Phytoplankton in the tropics, in contrast, tend to be tiny cyanobacteria, which are eaten by tiny zooplankton, which are eaten by slightly larger ones and so on. There are several more links in the food chain – and 90% of the energy is lost at each link. This is part of the reason why tropical waters tend to support fewer fish, and thus less vigorous fisheries, than cold waters.

As the oceans get warmer, some cold-water regions are shifting towards the longer-food-chain regime. In the North Atlantic, the boundary between the two types of food chain has already shifted 1000 kilometers north in recent decades.